British police will investigate the deaths of Nikolai Glushkov, left, Boris Berezovsky, centre, and Badri Patarkatsishvili, who were all opponents of President PutinDemyan Kudryavtysev/east2west

Russia warned Britain not to issue threats and “groundless ultimatums” as it defied a demand to account by midnight for the use of its nerve agent in the Salisbury poisoning.

Theresa May is expected to tell MPs today of plans to retaliate for the assassination attempt on a Russian double agent and his daughter last week. Initial targets are likely to include allies of President Putin who have assets in Britain, Russian diplomats based in London and the Kremlin-controlled television station RT.

Moscow’s embassy warned that any “punitive” action “will meet with a response” and described the ultimatum as a clear provocation. With relations close to breaking point:

• Counterterrorism police opened an investigation into the death at his south London home of Nikolai Glushkov, 68,…